Method for epitaxial growth of compound semiconductor using MOCVD with molecular layer epitaxy

ABSTRACT

A method for epitaxial growth of compound semiconductor containing three component elements, two component elements thereof being the same group elements, in which three kinds of compound gases each containing different one of the three component elements are cyclically introudced, under a predetermined pressure for a predetermined period respectively, onto a substrate enclosed in an evacuated crystal growth vessel so that a single crystal thin film of the compound semiconductor is formed on the substrate.

FIELD OF THE INVENTION

This invention relates to an epitaxial growth technique of a compoundsemiconductor and, more particularly, to a method for the epitaxialgrowth of a mixed crystal semiconductor of more than ternary alloys inwhich the thickness of the semiconductor is controllable in monolayeraccuracy

DESCRIPTION OF THE PRIOR ART

Various semiconductor devices using semiconductors of III and V groupelements such as, for example, a semiconductor having a hetero junctionbetween thin films of GaAs and Al_(x) Ga_(1-x) As, and semiconductorshaving hetero junctions such as HEMT structures or superlatticestructures utilizing the two-dimensional electron gas have hitherto beenproposed.

With such proposals for the semiconductor devices, the development ofexcellent techniques for the thin film crystal growth of compoundsemiconductors has been more urgent.

A molecular beam epitaxy (hereinafter referred to as MBE), a metalorganic vapor phase epitaxy (hereinafter referred to as MO-CVD) and amolecular layer epitaxy (hereinafter referred to as MLE) are well knownin the art as techniques for forming semiconductors or thin films ofsuch as GaAs or Al_(x) Ga_(1-x) As.

The MO-CVD method is widely used since the apparatus carrying out thismethod is simple and suitable for the mass production. In the MO-CVDmethod, however, the thickness of the semiconductor thin film to beformed can not be controlled in monolayer accuracy. Thus, the MO-CVDmethod is not necessarily suitable for manufacturing the HEMT structureand the superlattice structure.

In the MBE method, since the raw material of a crystalline thin filmbeing formed on a substrate crystal is heated and the vapor of the rawmaterial is deposited on the substrate, the growing rate of thecrystalline thin film can be kept very small so that the controllabilityof the thickness of the crystalline thin film is superior to that of theMO-CVD method. However, it is not easy to control the thickness of thethin film in monolayer accuracy. The problem is to overcome by using amonitoring according to the RHEED (reflection high energy electrondiffraction) method. Furthermore, to obtain a high quality crystal inthe MBE method, it is necessary to set up the growth temperature at550°-600° C. The growth temperature is normally set at a temperature of550°-600° C. for GaAs and even more than 600° C. for Al_(x) Ga_(1-x) As.However, the fact that Al tends to more easily oxidize at such a hightemperature leads to a serious defect that the formed crystal of Al_(x)Ga_(1-x) As has a poor flatness. Furthermore, if a steep impurityprofile is desired in such a formed crystal, the redistribution of theimpurity profile caused under such a high growing temperature would be aproblem. Moreover, since the MBE method is based upon the vapordeposition process, there may also be caused deviations from thestoichiometric composition of a formed crystal thin film orinterpositions of a crystal defect such as an oval defect in the formedcrystal thin film.

The molecular layer epitaxy is well known as a crystal growth process.For the crystal growth of a compound containing III and V groupelements, a compound gas containing a III group element and anothercompound gas containing a V group element are alternately introducedonto a substrate so that the crystal of the compound containing III andV group elements is grown monolayer by monolayer (see, for example J.Nishizawa, H. Abe and T. Kurabayashi; J. Electrochem. Soc. 132 (1985)1197-1200). This method utilizes the adsorption and the surface reactionof compound gases. In the case, for example, of forming a crystalcontaining III and V group elements, the growth of a single monolayer ofthe crystal is attained by introducing a compound gas containing the IIIgroup element and another compound gas containing the V group elementfor one period of time, respectively. Since the method utilizes themonolayer adsorption of the compound gases, the monolayer by monolayergrowth of the crystal is always attainable even though there is afluctuation of the pressure of the introduced compound gases. In thismethod, although trimethyl gallium (hereinafter referred to as TMG) asalkylgallium and arsine (AsH₃) as arsenic hydride have conventionallybeen used, a high purity GaAs can be grown at a lower temperature by thesubstitution of triethyl gallium (hereinafter referred to as TEG) forTMG as alkylgallium (see, for example J. Nishizawa, H. Abe, T.Kurabayashi and N. Sakurai; J. Vac. Sci. Technol. A4(3), (1986)706-710).

These methods such as described above relate, however, to the epitaxialgrowth of two-element compound semiconductors. In the epitaxial growthof ternary alloy semiconductors, the products superior in quality cannot be, therefore, obtained by these methods. In the case of the MLEmethod, particularly, atomic layers of a single kind of element X or Yare stacked, alternately, to form a compound XY, whereas the notion ofthe atomic layer of a single kind of element does no longer hold forA_(x) B_(1-x) in a mixed crystal A_(x) B_(1-x) C. For mixed crystalscontaining four elements or more, the situation is the same as the mixedcrystal containing three elements.

SUMMARY OF THE INVENTION

In view of the foregoing, it is the main object of the present inventionto provide a method for the epitaxial growth of compound semiconductorsof a mixed crystal based upon the notion of the monolayer growth whereinthe thickness of the epitaxially growing mixed crystal can be controlledin monolayer accuracy.

More specifically, it is an object of the present invention to provide amethod for the epitaxial growth of a mixed crystal thin film containingthree elements and more which is provided with a good reproducibilityand suitable for the mass production, wherein the thickness of theepitaxially growing thin film can be controlled to a precision ofseveral angstroms yet by a simple manner.

Also an object of the present invention is to provide a method for theepitaxial growth of a mixed crystal thin film containing three elementsand more, wherein the mixed crystal thin film may epitaxially be grownat a relatively low growth temperature by suitably selecting rawmaterials so that compound semiconductor devices of high quality may bemanufactured.

Other and further objects, features and advantages of the invention willappear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic view showing a crystal growth device for carryingout the epitaxial growth of Al_(x) Ga_(1-x) As layer according to thepresent invention.

FIG. 1B is a timing chart showing various gas introduction modes forepitaxial growth processes.

FIG. 2 is a graph showing a measured example of Al component in amultilayer structure of Al_(x) Ga_(1-x) As and GaAs.

FIG. 3A is a sectional view showing the structure of ametal-insulator-semiconductor (MIS) capacitor manufactured as a trialaccording to the present invention.

FIG. 3B is a characteristic diagram showing the relation between thecapacitance and the applied voltage of the MIS capacitor.

FIG. 4A is a schematic view showing a crystal growth device for carryingout the epitaxial growth of Al_(x) Ga_(1-x) As doped with impurities.

FIG. 4B is a timing chart showing gas introduction modes for epitaxialgrowth processes accompanied with impurity dopings.

FIG. 5 is a schematic view showing a crystal growth device for carryingout the epitaxial growth of Al_(x) Ga_(1-x) As accompanied with lightirradiation.

FIG. 6 is a schematic view showing a crystal growth device for carryingout the epitaxial growth of III-V and II-VI mixed crystal compoundsemiconductors containing three and four elements.

FIGS. 7 to 11 are schematic sectional views illustrating the structuresof ultrathin films obtained by the epitaxial growth according to thepresent invention.

FIG. 12A is a schematic view showing the construction of a crystalgrowth device automatically carrying out the epitaxial growth processesaccording to the present invention.

FIG. 12B is a schematic diagram showing the internal construction of acontrolling unit of FIG. 12A for controlling the introduction mode of acompound gas.

FIG. 12C is a timing chart showing a process carried out by the crystalgrowth device shown in FIG. 12A.

FIG. 13A is a schematic view showing the construction of another crystalgrowth device automatically carrying out the epitaxial growth processesaccompanied with light irradiation.

FIG. 13B is a timing chart showing a process carried out by the crystalgrowth device of FIG. 13A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the invention for the epitaxial growth ofAl_(x) Ga_(1-x) As single crystal thin film will now be described withreference to the schematic view in FIG. 1. A substrate crystal 7 of GaAsis mounted on a quartz susceptor 8 disposed in a crystal growth vessel11. The vessel 11 is coupled to an evacuating system 13 via a gate valve12 for evacuating its interior to an ultrahigh vacuum. The vessel 11 isalso provided with an infrared ray lamp 10 enclosed in a casing 9 forirradiating the GaAs substrate crystal 7. The vessel 11 is furtherprovided with three nozzle means 1, 2 and 3 for introducing alkylaluminum as a gaseous compound containing aluminum (Al), alkyl galliumas a gaseous compound containing gallium (Ga) and arsine (AsH₃) as agaseous compound containing arsenic (As), respectively. The nozzle means1, 2 and 3 are connected via respective controlling units (CTL's) 4, 5and 6 for controlling the introduced amount (per unit time) ofrespective gaseous compounds to their external sources, respectively.The controlling units 4, 5 and 6 are coupled to a controlling system 14for controlling the introduction mode of the various gaseous compounds.

A single Crystal thin film of Al_(x) Ga_(1-x) As is grown in a manner asdescribed below. First, the vessel 11 is evacuated to get a vacuum ofabout 10⁻⁹ to 10⁻¹⁰ Torr by opening the gate valve 12 and operating theevacuating system 13. The evacuating system 13 may be constituted with avacuum pump system comprising a combination of pump units such ascryopumps and molecular turbo pumps. Then, the GaAs substrate 7 isheated to a crystal growth temperature of 300° to 500° C. by theinfrared ray lamp 10 and the growth temperature is kept constant.Thereafter, alkyl aluminum as a gaseous compound containing aluminum,alkyl gallium as a gaseous compound containing gallium and arsine )AsH₃)as a gaseous compound containing arsenic are introduced in the vessel IIaccording to a manner that will be described, in more detail, below tocarrying out the epitaxial growth of Al_(x) Ga_(1-x) As single crystalwhile keeping the controllability of the thickness of Al_(x) Ga_(1-x) Asthin films within several angstroms.

Examples of the mode for introducing the three kinds of gaseouscompounds will now be described with reference to FIG. 1B in which threeexamples of the mode are shown. In each mode, on-off timings for theintroduction of each gaseous compound are controlled by the controllingsystem 14 while the introduced amount per unit time of each gaseouscompound is controlled by the controlling units 4, 5 and 6,respectively.

In a mode shown in FIG. 1B as mode I, gaseous alkyl gallium as a gascontaining Ga is first introduced for a period of t₁ after an evacuationperiod of t₀. Then, the vessel 11 is evacuated again by operating thegate valve 12. After a period of t₂, gaseous alkyl aluminum isintroduced as a gas containing Al for a period of t₃. Then, the vessel11 is evacuated again for a period of t₄. Thereafter, gaseous arsine(AsH₃) is introduced as a gas containing As for a period of t₅. As aresult of this one cycle of the operation which requires a total periodof t₆, a single crystalline thin film which has a thickness of severalangstroms is grown.

In this mode I, the crystal growth is repeatedly carried out by way oftrial under various conditions as follows:

t₀ =0 to 5 sec, t₁ =1 to 6 sec, t₂ =0 to 5 sec,

t₃ =1 to 6 sec, t₄ =0 to 5 sec, t₅ =5 to 20 sec and t₆ =7 to 47 sec.

In another mode shown in FIG. 1B as mode II, the same operation iscarried out except that the order of the introduction between a gascontaining Ga and a gas containing Al is interchanged.

In this mode II, the crystal growth is repeatedly carried out by way oftrial under various conditions as follows:

t₁₀ =0 to 5 sec, t₁₁ =1 to 6 sec, t₁₂ =0 to 5 sec,

t₁₃ =1 to 6 sec, t₁₄ =0 to 5 sec, t₁₅ =5 to 20 sec and t₁₆ =7 to 47 sec.

In still another mode shown as mode III in FIG. 1B, a gas containing Asis introduced after the introduction and the evacuation period of a gascontaining Al and, thereafter, the gas containing As is introduced againfollowing the introduction and evacuation periods of a gas containingGa.

In this mode III, the crystal growth is repeatedly carried out by way oftrial under various conditions as follows:

t₂₀ =0 to 5 sec, t₂₁ =1 to 6 sec, t₂₂ =0 to 5 sec,

t₂₃ =5 to 20 sec, t₂₄ =0 to 5 sec, t₂₅ =1 to 6 sec,

t₂₆ =0 to 5 sec, t₂₇ =5 to 20 sec and t₂₈ =12 to 72 sec.

In each mode described above, the internal pressure of the vessel 11 iskept at 10⁻⁶ to 10⁻⁴ Torr during the introducing period of gaseous alkylaluminum, 10⁻⁶ to 10⁻⁴ Torr during the introducing period of gaseousalkyl gallium and 10⁻⁵ to 10⁻³ Torr during the introducing period ofgaseous arsine.

Further, in each mode, one of materials such as trimethyl aluminum(TMA), triethyl aluminum (TEA) and triisobutyl aluminum (TIBA) is usedby way of trial as the raw material of gaseous alkyl aluminum. Also, oneof materials such as trimethyl gallium (TMG) and triethyl gallium (TEG)is used by way of trial as the raw material of gaseous alkyl gallium. Ineach case, arsine (AsH₃) is used as the raw material of the gascontaining As.

Characteristics of epitaxial thin layers of Al_(x) Ga_(1-x) As which areobtained in various cases such as described above will now be described.The carrier density in an epitaxial layer of Al_(x) Ga_(1-x) As which isformed by using TMA as alkyl aluminum and TMG as alkyl gallium is 10¹⁸to 10²⁰ cm⁻³ (p type) at room temperature. When TEG is used as alkylgallium, the carrier density of 10¹⁵ to 10¹⁸ cm⁻³ (p-type) is obtainedby the use of TEA as alkyl aluminum while the carrier density of 10¹³ to10¹⁵ cm⁻³ (p-type) is obtained by the use of TIBA as alkyl aluminum sothat an Al_(x) Ga_(1-x) As layer of higher purity may be obtained.

Further, when the three modes shown in FIG. 1B are carried out by usingthe same corresponding gaseous compounds and the same conditions for theinternal pressure of the vessel 11 during the introduction of eachgaseous compound in each mode, an epitaxial Al_(x) Ga_(1-x) As layerhaving the lowest carrier density may be obtained by the mode II. In acase in which TEG as alkyl gallium, TIBA as alkyl aluminum and AsH₃ areused, for example, an epitaxial layer having the carrier density of 10¹³to 10¹⁵ cm⁻³ (p-type) is grown in the mode II while a layer having thecarrier density of 10¹⁵ to 10¹⁶ cm⁻³ (p-type) is grown in the mode I.

In any case or mode, the thickness of a thin film of Al_(x) Ga_(1-x) Aswhich is grown in a single cycle of the introduction of gases is about 1to 10 Å. Thus, by suitably selecting the internal pressure of the vessel11 during the introducing period of each compound gas and the period oftime for introducing each compound gas, an epitaxial growth of Al_(x)Ga_(1-x) As thin film having the thickness corresponding to a singlemonolayer may easily be obtained in the single cycle of the introductionof gases. The thickness of one single monolayer is about 2.8 Å at (100)plane and about 3.3 Å at (111) plane.

Furthermore, any value within 0 to 1 for x the composition of Al inAl_(x) Ga_(1-x) As may also be obtained by suitably selecting theinternal pressure of the vessel 11 during the introducing period of eachcompound gas and the introducing period thereof.

The temperature of the crystal growth is kept at a value of 300° to 500°C. Since the redistribution of the impurity profile is suppressed due tosuch a rather lower growth temperature and, in addition, since thecontrollability for the thickness of the thin film being grown isexcellent, a steep impurity profile may be attained in a multilayer thinfilm comprising GaAs and Al_(x) Ga_(1-x) As.

FIG. 2 shows a depthwise profile of the composition x in Al_(x) Ga_(1-x)As included in a multilayer structure GaAs/Al_(x) Ga_(1-x)As/GaAs/Al_(x) Ga_(1-x) As . . . which is manufactured by using a groupof materials (TEG-AsH₃) and another group of materials (TEG-TEA-AsH₃).The composition x is measured by the Auger electron spectroscopy. Thethickness of a GaAs layer in this multilayer structure is attained in asimilar manner to such as described above by using the group ofmaterials (TEG-AsH₃) and repeating 100 cycles of the serial introductionof such compound gases. Also, the thickness of an Al_(x) Ga_(1-x) As inthis multilayer structure is attained by using the group of materials(TEG-TEA-AsH₃) and repeating 250 cycles of the serial introduction ofsuch compound gases. Conditions for such introduction of gases are setso as to obtain an x value of 0.36 to 0.37. As will be seen from thegraph shown in FIG. 2, a hetero junction between GaAs and Al_(x)Ga_(1-x) As which has a steep impurity profile may be obtained.

FIG. 3A shows a sectional view of a MIS (metal-insulator-semicondctor)capacitor manufactured as a trial by using a group of materials(TEG-TIBA-AsH₃). A layer 16 of Al_(x) Ga_(1-x) As is epitaxially grownat a temperature of 350° to 550° C. on a Si doped (3×10⁶ cm⁻³) n-typeGaAs substrate crystal 15. On the layer 16, an electrode 18 of Al isformed and a terminal 19 is attached thereto. On the bottom surface ofthe substrate 15, an electrode 17 of AuGe is formed and a terminal 20 isattached thereto. The thickness of the Al_(x) Ga_(1-x) As layer 16 isabout 700 Å and the diameter of the Al electrode 18 is 500 μm.

The capacitance to voltage (C-V) characteristic curve of the MIScapacitor is shown in FIG. 3B. It will be seen that a high purityepitaxial layer of Al_(x) Ga_(1-x) As may be obtained. It will also benoted that, according to the present invention, crystals of good qualitymay be grown at a lower growth temperature in comparison with priormethods such as MO-CVD or MBE.

FIG. 4A shows a schematic view of a device for manufacturing Al_(x)Ga_(1-x) As epitaxial layers of p-type and n-type by means of doping.One of gaseous compounds containing IV or VI group elements such asdisilane (Si₂ H₆), selenium hydride (H₂ Se), dimethyl selenium (DMSe)and diethyl tellurium (DETe) is introduced from an external sourcethereof via a controlling unit 33 for controlling the introduced amountof the gaseous compound and a nozzle 31 into a crystal growth vessel 11.Also, one of gaseous compounds such as dimethyl zinc (DMZn) and dimethylcadmium (DMCd) is introduced from an external source thereof via acontrolling unit 34 for controlling the introduced amount of the gaseouscompound and a nozzle 32 into the vessel 11. The introduction timing ofgases introduced through the nozzles 31 and 32 is controlled by acontrolling system 14 in a similar manner to that described withreference to FIG. 1A. Other parts and functions thereof are same as orcorrespond to that of FIG. 1A and the description will not be repeatedhere.

An epitaxial layer of Si doped n-type Al_(x) Ga_(1-x) As may, forexample, be formed by introducing Si₂ H₆ and that of Zn doped p-typeAl_(x) Ga_(1-x) As may be formed by introducing DMZn The gasintroduction modes for these cases are shown in FIG. 4B. These modes forthe above described cases in which TEG as a gaseous compound containingGa and TIBA as a gaseous compound containing Al are used by way ofexample will now be described.

In a mode IV of FIG. 4B which is the case that Si₂ H₆ is introduced,various gaseous compounds are introduced in the order of TIBA, TEG, Si₂H₆ and AsH₃. In this mode, any gaseous compound containing any III groupelement and a gaseous compound containing an impurity element which willoccupy the sites of III group atoms in a crystal are alternatelyintroduced. Such mode is most effective in the doping with the use ofSi₂ H₆ and an n-type Al_(x) Ga_(1-x) As (10¹⁶ to 10¹⁹ cm⁻³) layer may bemanufactured. Another mode V in FIG. 4B is the case in which DMZn isintroduced. In this case, the most effective doping is attained whenvarious gases are introduced in the order of DMZn, TIBA, TEG and AsH3 asshown in the mode V and a p-type Al_(x) Ga_(1-x) As (10¹⁶ to 10²⁰ cm⁻³)layer may be manufactured.

A gaseous compound containing an impurity may be introduced before andafter each introduction, in the order, of a gaseous compound containingIII_(A) group element, that containing III_(B) group element and thatcontaining V group element or, in the order, of V, III_(A) and III_(B).In case of III-V mixed crystal, main impurities are II, IV and VI groupelements. These impurities may be introduced in the order, II, III_(a),III_(B), IV, V and VI.

FIG. 5 shows a further form of the crystal growth device, which isdesigned to irradiate the substrate crystal during the crystal growth.The substrate 7 is irradiated with ultraviolet rays 23 and 24 which areemitted from external sources such as excimer lasers, argon ion lasers,xenon lamps or mercury lamps and transmitted through synthetic quartzwindows 21 and 22. As a result, the crystal growth temperature may bereduced to ensure growth of a single crystal having a still higherquality. Thus, for example, a high purity Al_(x) Ga_(1-x) As epitaxiallayer having a carrier density of about 10¹⁴ cm⁻³ may be obtained at alow growth temperature of 300° to 400° C. by using a group of materials(TEG-TIBA-AsH₃)

Further, the doping efficiency may be controlled by synchronizing anirradiation of light having a specific wave length with the dopingperiods such as the introduction periods of Si₂ H₆ or DMZn. Inaccordance with such purpose, two kinds of light 23 and 24 havingdifferent wave lengths may be available through the windows 21 and 22,respectively, to irradiate the substrate 7.

FIG. 6 shows a further embodiment of the crystal growth device formanufacturing quaternary alloys of III-V compound mixed crystals.

The manufacturing process, by way of example, for a quaternary alloyIn_(x) Ga_(1-x) As_(y) P_(1-y) of III-V compound semiconductors will nowbe described. In this process, gaseous alkyl indium is introduced froman external source via a controlling unit (CTL) 44 for controlling theintroduced amount of gaseous alkyl indium and a nozzle 41 into a crystalgrowth vessel 11. In like manners, gaseous alkyl gallium via acontrolling unit (CTL) 45 and a nozzle 42, and gaseous AsH₃ via acontrolling unit (CTL) 46 and a nozzle 43 are introduced, respectively,into the vessel 11. Also, gaseous phosphine (PH₃) as an impurity gas isintroduced from an external source thereof via a controlling unit 48 anda nozzle 47 into the vessel 11.

By alternately introducing these gases on a substrate crystal 7, anIn_(x) Ga_(1-x) As_(y) P_(1-y) single crystalline thin film may be grownat a low temperature of 300° to 500° C. while keeping thecontrollability of the growing film thickness within several angstroms.

Alternatively, a single crystalline thin film of Zn_(x) Se_(1-x) Te mayalso be grown at a low temperature of 300° to 500° C. in a similarmanner as described above by substituting dimethyl zinc (DMZn), seleniumhydride (H₂ Se) and dimethyl tellurium (DMTe) for alkyl indium, alkylgallium and AsH₃

Further, a single crystalline thin film of Hg_(x) Cd_(1-x) Te may alsobe grown by using gases of dimethyl mercury (DMHg), dimethyl cadmium(DMCd) and dimethyl tellurium (DMTe).

FIG. 7A is an embodiment according to the present invention in which amanufacturing process for a superlattice structure is schematicallyshown. In the superlattice structure shown in FIG. 7A, a GaAs layer 70having the thickness d₁ and an Al_(x) Ga_(1-x) As layer having thethickness d₂ are alternately and successively grown. According to theinvention, the thickness d₁ and d₂ may be controlled dimensionally asprecise as a monolayer. Thus, the minimum thickness in the superlatticestructure is that of a monolayer. Further, the thickness d₁ and d₂ mayeasily be controlled so as to an arbitrary number of times of a singlemonolayer thickness may be attained as required.

Furthermore, by using a doping process such as described above,superlattice structures such as an n-GaAs/n-Al_(x) Ga_(1-x) Assuperlattice structure and an n-GaAs/p-Al_(x) Ga_(1-x) As superlatticestructure may also be manufactured.

FIG. 7B shows the band structure of the n-GaAs/n-Al_(x) Ga_(1-x) Assuperlattice structure comprising a band structure 72 of n-GaAs and aband structure 73 of n-Al_(x) Ga_(1-x) As. Also. FIG. 7C shows the bandstructure of the n-GaAs/p-Al_(x) Ga_(1-x) As superlattice structurecomprising a band structure 72 of n-GaAs and a band structure 75 ofp-Al_(x) Ga_(1-x) As.

In the processes for manufacturing such superlattice structures, thethickness of each growth layer may be controlled in accordance with apredetermined design in like manner as the embodiment of FIG. 7A.Further, the doping may be carried out in the same manner such as shownin FIGS. 4A and 4B for Al_(x) Ga_(1-x) As, and in like manner of FIG. 4Bwithout Al for GaAs.

Further, an n-Al_(x) Ga_(1-x) As/p-Al_(x) Ga_(1-x) As superlatticestructure may also be manufactured according to the method shown inFIGS. 4A and 4B. Alternate doping of n-type and p-type impurities withor without changing the composition of Al in monolayer by monolayermanner may also be carried out. Of course, the thickness of n-Al_(x)Ga_(1-x) As and p-Al_(x) Ga_(1-x) As may be changed, respectively.

Embodiments of poly-type superlattice structures are shown in FIGS. 8Ato 8C. There are shown, by way of example, three those superlatticestructures each comprising a combination of three kind of semiconductorssuch as, for example, InAs 80, AlSb 81 and GaSb 82. Thus, FIG. 8A showsa superlattice structure comprising a unit periodic structure 80, 81 and82, FIG. 8B shows a superlattice structure comprising a unit periodicstructure 80, 81, 80 and 82, and FIG. 8C shows a superlattice structurecomprising a unit periodic structure 80, 82, 81 and 82. Thecombinations, conduction types of layers, the impurity density in eachlayer and the thickness of layers may be changed as required.

Since the superlattice structure is grown in monolayer by monolayermanner according to the method of the invention, ununiformity ofmicroscopic atom configuration is eliminated from the manufactured mixedcrystals so that the superlattice structure having a periodic structureproduced by a combination of ideal mixed crystals regularly arranged inatomic accuracy may be manufactured. Furthermore, since the crystalgrowth may be carried out at a low temperature of about 300° C., asuperlattice structure having a steep impurity profile may also bemanufactured with the use of impurity doping.

Referring now to FIG. 9, a process of the invention for manufacturing aHEMT (high electron mobility transistor) such as shown in FIG. 9 will bedescribed. Undoped GaAs layer 91 having a thickness of about 200monolayers is, first, grown on a semi-insulating GaAs substrate 90.Then, a Si doped Al_(x) Ga_(1-x) As (x≈0.3) layer 93 having a thicknessof 30 to 40 monolayers is grown. Thereafter, source, gate and drainelectrodes are formed. The source electrode 94 is an ohmic electrode ofAuGe/Au and the gate electrode 95 is a Ti/Pt/Au electrode. In themanufacturing process of the invention, since the ununiformity inmicroscopic configuration of atoms in the mixed crystals may beeliminated, the scattering, which may be occurred at the interfacebetween the n-Al_(x) Ga_(1-x) As layer 93 and the undoped GaAs layer 91,of electrons in a two-dimensional electron gas layer 92 is minimized sothat a highly efficient transistor may be manufactured. Of course, anydesigned values of thickness of layers, composition x and impuritydensities may be attained.

Another process of the invention for manufacturing a diode utilizing theresonance tunneling effect will now be described with reference to FIG.10 in which a schematic sectional view of such diode is shown. An n⁺-GaAs layer 101 having 100 monolayer thickness and an impurity densityof about 10⁸ cm⁻³ is, first, grown on an n⁺ -GaAs substrate 100.Thereafter, an Al_(x) Ga_(1-x) As high resistance layer 102 having 15 to20 monolayer thickness, an n-GaAs layer 103 having 15 to 20 monolayerthickness and an impurity density of 10¹⁷ cm⁻³, an Al_(x) Ga_(1-x) Ashigh resistance layer 104 having 15 to 20 monolayer thickness and an n⁺-GaAs layer 105 having 100 monolayer thickness and an impurity densityof cm⁻³ are successively grown. Finally, an ohmic contact 106 of suchas, for example, AuGe/Ni is formed on the top surface of the layer 105and the bottom surface of the substrate 100, respectively. Such a devicewill exhibit a negative resistance when a DC voltage is applied innormal or reversed direction at 4K to the room temperature so that thedevice is available as an oscillator, a mixer or a detector inmillimeter wave and submillimeter wave bands.

Still another process of the invention for manufacturing a transistor inwhich the resonant tunneling structure such as described above isdisposed in the channel of a field effect transistor will now bedescribed with reference to FIG. 11 in which a schematic sectional viewof such transistor is shown.

In the drawing, like reference characters designate like parts as shownin FIG. 10 and the description thereof will not be repeated. In thechannels of a drain region 110 and a source region 111, the impuritydensity may be designed to have a value less than about 10¹⁶ cm⁻³ sothat the velocity of electrons may not be affected due to the latticescattering in these regions. Further, a source electrode 112 and a drainelectrode 113 are formed by Au-Ge/Ni/Au while a gate electrode 114 isformed with a Schottky gate of such as Al, Pt, Mo and Cr or an ohmicelectrode of such as well known Au-Ge/Ni/Au.

The thickness of the drain region 110 and the source region 111 may bedesigned in such a manner that the total thickness of the drain side isthicker than that of the source side so as to keep the withstand voltagebetween gate and drain larger than that between source and gate.Further, in the normal operation of the transistor, the thickness W ofthe drain region may be designed by considering the cut-off frequencyf_(c) as the criterion. Thus, when f_(c) is, for example, 10 GHz, 100GHz, 1000 GHz (=1 THz) and 10 THz, respectively, W may be designed asabout 100 μm, 1 μm, 1000 Å and 100 Å, respectively.

Also, when the transit time effect of gate to drain is utilized, thethickness of the drain region may be designed so as to result thetransit angle θ of 3π/2 if the time constant between gate and source isnegligible and the maximum injection at a phase of π/4 is intended whilethat of the drain region may be designed so as to result the transitangle of π/2 to π if the time constant between gate and source is nolonger negligible and the maximum injection at a phase of π/2 to π isintended. When θ=π, then f_(osc) max =V_(s) /2W where V_(s) and W arethe carrier velocity and the thickness of the drain region,respectively. Thus, in order to set f_(osc) max as 100 GHz, 1000 GHz (=1THz) or 10 THz, W may be chosen as 0.5 μm, 500 Å or 50 Å, respectively.Of course, the thickness of the resonance tunneling parts 102 to 104 isreduced corresponding to the increasing f_(c) 's The impurity density inn⁺ -GaAs layers 101 and 105 may be designed as more than 10¹⁸ cm⁻³ whilethat in the resonance tunneling junction may be designed as same as thatin FIG. 10 or a predetermined value.

It is apparent that the crystal growth method of the invention mayequally well be embodied for manufacturing ultra thin film structuresother than the embodiments such as described above with reference toFIGS. 7 to 11. Also, the embodiments has been described above referringto GaAs and Al_(x) Ga_(1-x) As as growth layers. The process of theinvention, however, may also be applied to such as other III-V compoundsand mixed crystals thereof, II-VI compounds and mixed crystals thereof,and hetero-junctions between III-V and II-VI compounds.

In the above described embodiments of FIGS. 7 to 11, although varioussuperlattice structures and hetero-junctions may be grown in themonolayer by monolayer manner, it is necessary to control thetemperature of the substrate, the pressure and the introduction periodsof gaseous compounds containing component elements and impurity elementsas adjustable parameters.

Although only the controlling means for the introduction of variousgases has already been described with reference to FIGS. 1, 4, 5 and 6,another embodiment of the invention in which a desired ultra thin filmstructure may automatically be carried out will now be described.

FIGS. 12A is a schematic block diagram of an epitaxial growth deviceincluding controlling parts thereof for carrying out the processaccording to the invention.

The device comprises a crystal growth vessel 150, a gate valve 151, anevacuating system 152, a driving means 153 for driving the gate valve151, a vacuum gauge 154, a quartz susceptor 155 for mounting asemiconductor substrate 156, a lamp 158 for heating the semiconductorsubstrate 156 and a lamp housing 157. Most of these parts correspond tothose parts shown in FIGS. 1A, 4A, 5 and 6.

The heating temperature of the lamp 158 is controlled by a lamptemperature controlling unit 159 comprising such a power supply and athermostat. The temperature of the substrate 156 is observed by aradiation thermometer or pyrometer 161 through a window 160. An output162 from the thermometer is transmitted with an output 163 formonitoring the lamp heating temperature to a temperature controllingunit 165 for controlling the temperature of the substrate from which asignal 164 is transmitted to the unit 159. The internal pressure of thevessel 150 is measured by the vacuum gauge 154 and an output signal 168of the vacuum gauge 154 is transmitted to a controlling unit 166 forcontrolling the evacuating system 152. A controlling signal 167 from theunit 166 is transmitted to the evacuating system 152 and the drivingmeans 153. Gaseous compounds 170, 171, 172, 173 and 174 containing oneof component elements or impurity elements, respectively, are introducedvia respective stop valve 180, 181, 182, 183 and 184, respectivecontrolling unit 190, 191, 192, 193 and 194, and respective nozzle 200,201, 202, 203 and 204 into the vessel 150. Manually operated valves orelectromagnetic valves may be available for the valves 180, 181, 182,183 and 184. Also, the controlling units 190, 191, 192, 193 and 194control the pressure and the introducing periods of each gaseouscompound, respectively. The manner to control the pressure and theintroduction periods of each gaseous compound will, hereinafter, bedescribed more specifically with reference to FIG. 12B. Input signals tocontrol the units 190, 191, 192, 193 and 194, respectively, and outputmonitoring signals thereof are transmitted via signal lines 210, 211,212, 213 and 214, respectively. A controlling system comprising acomputer CPU 220, an input terminal 221, a memory 222, an image outputterminal 23 and an output printer operating system not shown isconnected through an interface circuit 215 to the group of units 190,191, 192, 193 and 194. In response to a monitoring input signal 233, thecomputer transmits a controlling signal 232. Also, in response to avacuum monitoring input signal 231, the computer transmits a controllingsignal 230. Further, in response to a temperature monitoring inputsignal 235, the computer transmit a controlling signal 234.

FIG. 12B shows a schematic block diagram illustrating a part of the gascontrolling unit. Since the manner of the operation of various units 190to 194 is the same, the description will be limited only to theintroduction of one gaseous compound. Thus, the operation, for example,of the unit 190 will be described.

The introduction of the gaseous compound 170 is controlled by a stopvalve 251 controlling introducing and non-introducing periods inresponse to a control signal transmitted from the computer through theline 210. The pressure of the gaseous compound 170 is controlled by amass flow controller 252 in response to an output controlling signal 257from a pressure controlling circuit 255. The pressure controllingcircuit 255 is an electronic circuit comprising a feedback control usingsuch as well known transistors, diodes and IC's. The circuit 255generates the output controlling signal 257 in response to an outputsignal 254 from a manometer 253 and a controlling signal 256 generatedfrom the computer in response to the output signal 254 of the manometer253.

Referring now to FIG. 12C, there is shown a mode for the introduction ofvarious gases together with a temperature controlling diagram which maybe used for carrying out a crystal growth process in which n-GaAs layersand p-Al_(x) Ga_(1-x) As layers are alternately grown on an n-GaAssubstrate in monolayer by monolayer manner by means of the controllingsystem described above. During a period of t₅₀, a single monolayer ofn-GaAs may be grown at a temperature T₅₀ and, subsequently, a singlemonolayer of p-Al_(x) Ga_(1-x) As may be grown in a period of t₅₁temperature T₅₁.

Of course, an epitaxial layer of such as 100 or 1000 monolayer thicknessmay also be grown. In such a case, a program, in an appropriatelanguage, instructing 100 or 1000 monolayer thickness may be input tothe computer. Thus, any desired ultrathin film structure may be grown.Since a crystal growth of a layer, for example, of two monolayerthickness may be carried out by adjusting the condition, such manner maybe available for the save of growth time.

Further, by using the program shown in FIG. 12C as a monitoring input todisplay on a Braun tube (CRT) as the image output terminal 223, a visualmonitoring may be carried out. Also, memory devices such as a floppydisk, a cassette tape recorder and a hard disk may be used as the memory222 for recording and reproducing the program of the process formanufacturing the ultrathin film structure.

Furthermore, consecutive crystal growth may be carried out by using anautomatic transport device for the insertion and take out operations ofthe substrate into and out of the crystal growth vessel. Thus, the massproduction of an ultrathin film comprising layers of differentstructures may be carried out by using a controlling program such asdescribed above in combination with a program for controlling theautomatic transport device.

Still further, since the monolayer growth may be carried out in a simplemanner by using a simple manometer and a controlling system forcontrolling the gas introduction and the substrate temperature, highquality wafers comprising ultrathin epitaxial layers may easily bemanufactured in contrast to the MBE method.

In the computer of FIG. 12A, there is shown only one input terminal.However, an operating system of multitask or multijob type comprising aplurality of input terminals to the computer may also be used forcarrying out the crystal growth control while programming other newprograms for the crystal growth.

Still another embodiment of the invention, in which the irradiation ofthe substrate with light during the crystal growth such as shown in FIG.5 is automated, will now be described.

Referring now to FIG. 13A, parts relating to the irradiation not shownin FIG. 12A will, in particular, be described. Ultraviolet rays 280 and281 emitted from ultraviolet ray sources 273 and 276 such as mercurylamps, excimer lasers and argon ion lasers, respectively, are introducedvia shutters 271 and 275, and windows 270 and 274, respectively, to asubstrate 156. The shutters 271 and 275 are operated by controllingsignals 293 and 294 through an interface circuit 292.

A manner of the irradiation with light 1 and light 2 which isaccompanied with the crystal growth process shown in FIG. 12C is shownin FIG. 13B. The irradiation with light 1 is continued before theintroduction of compound gas containing As is started and theirradiation with light 2 follows. The irradiation with light 2 iscontinued before the introduction of DMZn gas. Wave lengths of light 1and light 2 may be selected so as to activate the surface reactionduring the irradiation. Such a case may also be possible that light 1and light 2 have the same wave length but different intensities. Also,light having a range of wave lengths may be used for light 280 and 281,respectively. Further, two or more kinds of light having different wavelengths may be substituted for light 280 and 281, respectively, so as toirradiate the substrate with different kinds of light corresponding tothe introduction or non-introduction periods of different kind of gasesthereby more effective crystal growth may be carried out.

When a light source having a wide range of wave lengths such as amercury lamp is used, the irradiation of semiconductor substrate withone light source but two kinds of light having two different range ofwave lengths may be carried out by using a shutter with a filter.

While III-V mixed crystals have been referred to by way of example, theinvention is of course also applicable to II-VI mixed crystal, compoundsemiconductors containing four elements or heter-junctions between III-Vmixed crystals and II-VI mixed crystals merely by additionallyinstalling more gas introduction units to the embodiments shown in FIGS.12 and 13 as desired.

According to the present invention, the crystal growth of mixed crystalshaving ultrathin film structure with impurity doping, which has beendifficult in the prior art, may be carried out in monolayer accuracy. Inthe prior art such as the MBE method, it is necessary to use a complexand expensive controlling device such as RHEED for controlling thethickness of growing crystalline thin layers in high accuracy. Incontrast to the prior art, the process of the invention such asdescribed above may be automated in a simple manner by an inexpensivecontrolling device for controlling the pressure of gases, theintroduction of gases, the temperature of the substrate and theirradiation with light. Thus, the crystal growth method of the inventionis suitable for the mass production of ultrathin film structures in highaccuracy and effective in the industrial utilization.

Thus, according to the method of the invention, various devices such ashetero-junction devices, HEMT structure devices, superlattice structuredevices, two or three terminal devices, negative resistance devicesutilizing the transit time effect, tunnel injection devices, opticaldetector or light emitting devices, and semiconductor lasers may bemanufactured.

What is claimed is:
 1. A method of epitaxial growth for forming on asubstrate crystal a single crystalline thin film of a compoundsemiconductor A_(x) B_(1-x) C containing three component elements A, Band C, two elements A and B being elements of the same group, comprisingthe steps of successively introducing a plurality of gaseous compounds,each containing one of said three elements A, B and C, into the crystalgrowth vessel, evacuating the crystal growth chamber between thesuccessive introductions of two gaseous compounds so as to avoid mixingof the gaseous compounds together, and repeating a sequence of the abovesteps.
 2. A method according to claim 1, wherein each one of said threeelements is a III group element or a V group element, respectively.
 3. Amethod according to claim 1, wherein each one of said three elements isa II group element or a VI group element, respectively.
 4. A methodaccording to claim 1, wherein said three elements A, B and C are Al, Gaand As or Ga, Al and As, respectively, and a gaseous compound containingAl, a gaseous compound containing Ga and a gaseous compound containingAs are sequentially introduced, in the order of A, B, and C, into thecrystal growth vessel.
 5. A method according to claim 1, wherein saidthree elements A, B and C are Al, Ga and As, respectively, and gaseoustriethyl gallium is used as a gaseous compound containing Ga whilegaseous triethyl aluminum or gaseous trisobutyl aluminum is used as agaseous compound containing Al and the successively introducing step iscarried out in the order Al, Ga and As.
 6. A method of epitaxial growthfor forming on a substrate crystal a single crystal of a compoundsemiconductor doped with an impurity element comprising the steps ofsuccessively introducing into a crystal growth vessel a gaseous compoundIII_(A) containing a III group element as one of component elements ofthe compound semiconductor, a gaseous compound III_(B) containinganother III group element as another component element of the compoundsemiconductor, a gaseous compound V_(C) containing a V group element asstill another component element of the compound semiconductor and adoping gas containing the impurity element of the compound semiconductorin this order or in the order V_(C), III_(A), III_(B) and the dopinggas, and evacuating the chamber between the successive introductions oftwo gaseous compounds so as to avoid mixing together the gaseouscompounds.
 7. A method according to claim 6, wherein the two III groupelements and the V group element are Ga, Al and As, respectively, andsaid impurity element contained in the doping gas is an element of II,IV, or VI group.
 8. A method according to claim 7, wherein said dopinggas is disilane (Si₂ H₆), dimethyl zinc, dimethyl cadium, trimethylgallium, selenium hydride (H₂ Se), diethyl selenium or diethyltellurium.
 9. A method of epitaxial growth for forming on a substratecrystal of a single crystal of an impurity doped compound semiconductorcontaining two III group elements and a V group element as componentelements, and a II group element, a IV group element and a VI groupelement as impurity elements, the method comprising the step ofsuccessively introducing a gaseous compound III_(A) containing one ofthe two III group elements, a gaseous compound III_(B) containinganother one of the two III group elements, a gaseous compound V_(C)containing the V group element, a gaseous compound II_(D) containing theII group element, a gaseous compound IV_(E) containing the IV groupelement and a gaseous compound VI_(F) containing the VI group elementinto a crystal growth vessel in the order II_(D), III_(A), III_(B),IV_(E), V_(C) and VI_(F).
 10. A method of epitaxial growth for forming asubstrate crystal a single crystal of a quaternary alloy or a mixedcrystalline compound semiconductor containing four component elementssuch as two III group elements and two V group elements, the methodcomprising the step of successively introducing into a crystal growthvessel in the following sequence, a gaseous compound III_(A) containingone element of the two III group elements, a gaseous compound III_(B)containing another element of the two group elements, a gaseous compoundV_(C) containing one element of the two V group elements, and a gaseouscompound v_(D) containing another element of the two V group elements .11. A method according to claim 10, wherein said gaseous compoundIII_(A), III_(B), V_(C) and V_(D) is alkyl indium, alkyl gallium, arsine(AsH₃) and phosphine (PH₃), respectively.
 12. A method according to anyone of claims 1 to 11, wherein the substrate crystal is irradiated withultraviolet rays during the crystal growth.
 13. A method of epitaxialgrowth for forming on a substrate crystal a single crystal of a compoundsemiconductor containing three component elements, two elements thereofbeing elements of the same group but another one being an element ofdifferent group, the method comprising the steps of successivelyintroducing into a crystal growth vessel three kinds of gaseouscompounds respectively containing a different one of said threecomponent elements, evacuating the vessel between the successiveintroductions of two gaseous compounds so as to avoid mixing togetherthe gaseous compounds, and repeating the step of successivelyintroducing to form a multilayer thin film structure comprising a mixedcrystal of two different group elements and a mixed crystal of saidthree component elements.
 14. A method according to claim 13, whereinboth of said mixed crystal of two different group elements and saidmixed crystal of three component elements are II-IV mixed crystals. 15.A method according to claim 13, wherein said two different groupelements are Ga and As while said three component elements are Ga, Aland As.
 16. A method according to claim 15, wherein triethyl aluminum ortriisobutyl aluminum is used as one of said three kind of gaseouscompound containing Al while triethyl gallium is used as another one ofsaid three kind of gaseous compound containing Ga.
 17. A methodaccording to claim 16, wherein impurities such as disilane (Si₂ H₆),trimethyl gallium, selenium hydride (H₂ Se), diethyl selenium, diethyltellurium, dimethyl cadmium and dimethyl zinc are used.
 18. A methodaccording to claim 1, wherein the step of sequentially introducingincludes maintaining the pressure in the crystal growth vessel within10⁻³ Torr and 10⁻⁶ Torr during each introduction of the gaseouscompounds.
 19. A method according to claim 1, wherein the step ofsequentially introducing includes introducing gaseous alkyl aluminum ata maintained pressure of between 10⁻⁶ and 10⁻⁴ Torr, introducing gaseousalkyl gallium at a maintained pressure between 10⁻⁵ and 10⁻⁴ Torr, andintroducing gaseous arsine at a maintained pressure of between 10⁻⁵ to10⁻³ Torr.
 20. A method according to claim 1, further comprising thesteps of:impurity doping after the introduction of element C during thestep of successively introducing.
 21. A method according to claim 20,wherein the step of impurity doping includes doping with impuritiesselected from the group consisting of disilane (Si₂ H₆), trimethylgallium, selenium hydride (H₂ Se). dimethyl selenium, dimethyltellurium, dimethyl cadmium and dimethyl zinc.
 22. A method according toclaim 1, wherein the step of sequentially introducing includesintroducing the gaseous compounds in an order of one of A, B and C andA, C, B and C.